Xanthine alkaloids, which include caffeine, theophylline, and theobromine, are ubiquitously distributed in plants, such as the seeds of Coffea arabica and related species, the leaves of Thea sinensis, the seeds of Theobroma cacao, the nuts of the tree Cola acuminata, and the like. Extracts of these naturally occurring substances have been used throughout history as beverages and the pharmacologically significant nervous system stimulant properties of such concoctions have long been recognized.
Xanthine, itself, is 3,7-dihydro-1H-purine-2,6-dione. Chemically, therefore, xanthine and its derivatives are structurally related to uric acid and purine. Caffeine (1,3,7-trimethylxanthine), theophylline (1,3-dimethylxanthine), and theobromine (3,7-dimethylxanthine) represent the alkaloids most frequently associated with the expression "xanthine." However, numerous other xanthine derivatives have been isolated or synthesized. See, for example, Bruns, Biochem. Pharmacol., 30, 325-333 (1981), which describes more than one hundred purine bases and structurally related heterocycles with regard to adenosine antagonism, and Daly, J. Med. Chem., 25(3), 197-207 (1982).
Pharmacologically, the xanthines represent an important class of therapeutic agents. Observed pharmacological actions include stimulation of the central nervous system, relaxation of smooth muscle constrictions of the smaller bronchi and other smooth muscles, dilation of the small pulmonary arteries, stimulation of cardiac muscle with increased cardiac output, and the promotion of mild diuresis. Available evidence indicates that the therapeutic actions of these drugs involve blockade or antagonism of adenosine receptors.
It now has been recognized that there are not one but at least two classes of extracellular receptors involved in the action of adenosine. One of these has a high affinity for adenosine and has been found to be coupled to a number of secondary messenger systems, including inhibition of adenylate cyclase, inhibition of calcium entry, stimulation of potassium flux, and phosphoinositide metabolism (Van Galen et al., Medicinal Res. Rev., 12, 423-471 (1992)). This class has been termed by some as the A.sub.1 receptors. The other class of receptors has a low affinity for adenosine and has been found to elicit a range of physiological responses, including the inhibition of platelet aggregation (Lohse et al., Naunyn Schmiedeberg's Arch. Pharmacol., 337, 64-68 (1988)), dilation of blood vessels (Ueeda et al., J. Med. Chem., 34, 1340-1344 (1991)), erythropoietin production (Ueno et al., Life Sciences, 43, 229-237 (1988)), and depression of locomotor activity (Nikodijevic et al., J. Pharm. Exp. Therap., 259, 286-294 (1991)). This class has been termed the A.sub.2 receptors.
Subtypes of A.sub.2 receptors also have been identified. For example, A.sub.2a receptors, which are linked via G.sub.S guanine nucleotide binding proteins to the stimulation of adenylate cyclase, are present in high density in the striatum of the CNS. They are also present on platelets, pheochromocytoma cells, and smooth muscle cells. A.sub.2b receptors (Bruns et al., Mol. Pharmacol., 29, 331-346 (1986)) are found in the brain, fibroblasts, and intestines (Stehle et al., Mol. Endocrinol., 6, 384-393 (1992)).
Characterization of the adenosine receptors is now possible with a variety of structural analogues. Adenosine analogues resistant to metabolism or uptake mechanisms have become available. These are particularly valuable, since their apparent potencies are less affected by metabolic removal from the effector system than other adenosine analogues. The adenosine analogues exhibit different rank order of potencies at A.sub.1 and A.sub.2 adenosine receptors, providing a simple method of categorizing a physiological response with respect to the nature of the adenosine receptor. The blockade of adenosine receptors, i.e., antagonism, provides another method of categorizing a response with respect to the involvement of adenosine receptors.
Adenosine, perhaps, represents a general regulatory substance, since no particular cell type or tissue appears uniquely responsible for its formation. In this regard, adenosine is unlike various endocrine hormones. Furthermore, there is no evidence for storage and release of adenosine from nerve or other cells. Thus, adenosine is unlike various neurotransmitter substances.
Although adenosine can affect a variety of physiological functions, particular attention has been directed over the years to those functions that might lead to clinical applications. Preeminent has been the cardiovascular effects of adenosine, which lead to vasodilation and hypotension but which also lead to cardiac depression. The antilipolytic, antithrombotic, and antispasmodic actions of adenosine have also received some attention. Adenosine stimulates steroidogenesis in adrenal cells, probably via activation of adenylate cyclase, and inhibits neurotransmission and spontaneous activity of central neurons. Finally, the bronchoconstrictor action of adenosine and its antagonism by xanthines represents an important area of research.
Although theophylline and other xanthines, such as caffeine, are relatively weak adenosine antagonists, having affinity constants in the range of 10-50 micromolar, they owe many of their pharmacological effects to blockage of adenosine-mediated functions at the A.sub.1 and A.sub.2 receptor sites. The A.sub.1 -adenosine receptor is inhibitory to adenylate cyclase and appears involved in antilipolytic, cardiac, and central depressant effects of adenosine. The A.sub.2 -adenosine receptor is stimulatory to adenylate cyclase and is involved in hypotensive, antithrombotic, and endocrine effects of adenosine. Some xanthines, such as 3-isobutyl-1-methylxanthine, not only block adenosine receptors but also have potent inhibitory effects on phosphodiesterases.
The brochodilator effects of the xanthines, particularly, theophylline, have received considerable commercial attention and various preparations of theophylline, such as the anhydrous base or salts thereof, including sodium acetate, sodium benzoate, sodium salicylate, calcium salicylate, etc., are available as tablets, capsules, and elixirs including sustained released forms. Other related xanthines, such as dyphyllin, have received widespread usage. Caffeine has been used alone and in combination with other drugs in the treatment of headaches.
Many of the xanthines, however, such as theophylline, have undesirable side effects. Some of these side effects may be due to actions at sites other than adenosine receptors. It is also likely that some side effects are associated with blockade of the adenosine receptors, themselves. It appears that at least some of the side-effects caused by the adenosine receptor antagonists could be avoided by the development of more potent blockers of such receptors which, because of their increased blocking action, could be employed in lower doses and, thus, would be less likely to produce side-effects not associated with the adenosine receptor blockade. Additionally, where the therapeutic effect is due to blockade of one subtype of adenosine receptor, while side-effects relate to blockade of a different subtype of adenosine receptor, drugs, which are extremely potent at one receptor and substantially less active at another adenosine receptor, also should have a reduced likelihood of side-effects.
Potent and A.sub.2 -selective adenosine antagonists, suitable as pharmacological tools, have long been lacking. A.sub.2 -selective antagonists also may have application as therapeutic agents, e.g., in the treatment of Parkinson's disease (Schiffman et al., Drug Dev. Res., 28, 381-385 (1993)). The slightly selective, non-xanthine antagonist CGS 15943 was under development as an antiasthmatic (Jacobson et al., J. Med. Chem., 35, 407-422 (1992)). A low affinity antagonist, 3,7-dimethyl-1-propargylxanthine (DMPX), was reported to be A.sub.2 -selective but by less than one order of magnitude (Ukena et al., Life Sci., 39, 743-750 (1986)). It was relatively weak in blocking the in vivo effects of N.sup.6 -cyclohexyladenosine (CHA) compared to those of 5'-N-ethylcarboxamidoadenosine (NECA), suggesting some A.sub.2 selectivity. Several non-xanthine antagonists of the triazoloquinazoline class, including CGS 15943, are A.sub.2 -selective but also by only one order of magnitude (Francis et al., J. Med. Chem., 31, 1014-1020 (1988)). The locomotor activity of several members of this class was described previously (Griebel et al., NeuroReport, 2, 139-140 (1991)). A triazoloquinozaline derivative, CP66,713, was found to be 12-fold selective in binding assays at rat brain A.sub.2a - vs. A.sub.1 -receptors (Sarges et al., J. Med. Chem., 33, 2240-2254 (1990)). Low selectivity, interspecies differences in affinity, and low water solubility precluded extensive use of this compound. In one study, partial antagonism of A.sub.2 depression of locomotor activity was achieved in vivo using CP66,713 (Nikodijevic et al., 1991, supra). At the same dose CP66,713 had no effect on A.sub.1 depression of locomotor activity.
It was only recently that 8-styrylxanthines were reported as the first potentially useful compounds by Shimada et al. (J. Med. Chem., 35, 2342-2345 (1992)). These authors found that 8-styryl derivatives of 1,3-dimethylxanthines were the most selective for A.sub.2 receptors (selectivities greater than 5000-fold were reported), but the affinities of the corresponding 1,3-propyl analogues at both subtypes were greater (the most potent compound having a K.sub.i value of 7.8 nM at A.sub.2 receptors).
The literature is replete with examples of 8-substituted xanthine derivatives, including 8-substituted 1,3,7-trialkyl-xanthines, such as 8-styryl-1,3,7-trialkyl-xanthines. For example, U.S. Pat. No. 3,641,010 (Schweiss et al.) discloses 1,3-dialkyl-7-methyl-8-styryl-xanthines and describes the compounds as cerebral stimulants of the caffeine type. WO 92/06976 discloses alkyl-substituted 8-styryl-xanthines as selective A.sub.2 -adenosine receptor antagonists useful in the treatment of asthma and osteoporosis. 1-methyl-3,7-disubstituted-8-benzyl-xanthine derivatives useful in the treatment of asthma and bronchitis are disclosed in European Patent Application 0 215 736. The administration of methylxanthines, which are described as adenosine antagonists, to alleviate asystole and cardiac arrhythmia associated with resuscitation is described in U.S. Pat. No. 4,904,472. Various substituted theophyllines/xanthines are disclosed in U.S. Pat. Nos. 2,840,559, 3,309,271, 3,624,215, 3,624,216, 4,120,947, 4,297,494, 4,299,832, 4,546,182, 4,548,820, 4,558,051, 4,567,183, and 4,883,801, although only the U.S. Pat. Nos. 4,593,095, 4,612,315, 4,696,932, 4,755,517, 4,769,377, 4,783,530, 4,879,296, 4,981,857, 5,015,647, and 5,047,534 describe the disclosed compounds as potent adenosine receptor antagonists. Although a number of these references disclose xanthine compounds and describe them as "potent" and/or "selective" A.sub.2 -adenosine receptor antagonists, the potency and/or selectivity actually realized is not that significant. Accordingly, there remains a need for highly selective and potent A.sub.2 -adenosine receptor antagonists. Such compounds would reduce, if not completely eliminate, the side effects associated with A.sub.2 -adenosine receptor antagonists of reduced potency or selectivity by increasing blocking activity at one receptor, significantly, if not completely, eliminating blocking activity at non-A.sub.2 -adenosine receptors and, consequently, enabling the employment of reduced dosages.
An object of the present invention is to provide A.sub.2 -adenosine receptor antagonists of high potency and/or selectivity. Another object of the present invention is to provide a pharmaceutical composition comprising one or more of the present inventive adenosine receptor antagonists. Yet another object of the present invention is to provide a method of selectively antagonizing A.sub.2 adenosine receptors in a mammal in need of selective antagonism of its A.sub.2 adenosine receptors. By means of these objects, the present invention offers advantages over currently available A.sub.2 -adenosine receptor antagonists by providing A.sub.2 -selective adenosine receptor antagonists of increased potency and/or specificity. Accordingly, the present invention also provides an improved pharmaceutical composition comprising A.sub.2 -selective adenosine receptor antagonists and an improved method for the selective antagonism of A.sub.2 adenosine receptors in a mammal in need of such selective antagonism. The method, since it involves the use of A.sub.2 -selective adenosine receptor antagonists having increased potency and/or selectivity over currently available antagonists, is expected to reduce, if not completely eliminate, the side effects associated with the A.sub.2 -adenosine receptor antagonists by enabling the employment of reduced dosages.